Although the vapor compression distillation process is recognized as thermodynamically very efficient, the development of the vapor compression process has lagged behind multiple effect flash evaporation. The lack of usage of vapor compression distillers on a large scale is primarily the result of the limitations of mechanical compressors.
Most small vapor compression distillers employ rotary positive displacement compressors of the Roots type. These compressors are designed for low pressure differential and they are limited to approximately 10 psi compression. In practice, because of high heat build-up and slippage losses they are generally limited in use to employment for compression up to approximately 6 psi.
Large vapor compression systems employ centrifugal displacement compressors in single or multi-stage arrangement. Single stage compressors are limited to low pressure differential, requiring large heat exchanger surface. Multi-stage compressors operate on higher pressure differential, but because of high temperature rise due to adiabatic compression the efficiency is not good and heat build-up produces mechanical problems.
Liquid ring compressors have been employed on small vapor compression distillers, but their efficiency is not good, and they are limited in operation to low pressure differential because the liquid compression medium is constantly being pressurized and depressurized causing alternate internal condensing and flashing of vapor. For a given displacement the physical size of liquid ring compressors is large in comparison with other types.
Piston compressors are seldom used because of large mass, slow speeds, and poor efficiency. Sliding vane and other types of compressors requiring internal lubrication cannot be used in many applications because the presence of oil in the vapor has a detrimental effect on heat transfer, in addition to contaminating the product distillate.